Semester

Spring

Date of Graduation

2000

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Keh-Minn Chang.

Abstract

This research work investigates primarily the composition effects on solidification macrosegregation, i.e. freckles, in superalloys. First, the freckling mechanism of superalloys is developed using the physical simulation of freckle formation in specially designed model alloys. It is found that freckles originate from dendrite irregularities at the solidification front. The freckle channels flow downward in Nb-containing alloys and upward in T-containing alloy under horizontal solidification conditions. The horizontal solidification correctly simulates the horizontal component in remelt ingot where freckling potential is the highest. Based on the result, key control parameters for freckling are identified.;Thermodynamic simulation approach is employed to study the solidification behavior of superalloys. The theoretical prediction yields relative good agreement with experimental observations of freezing range, solidification sequence, and the occurrence of secondary phases. Solidification diagram of Ni-Cr-Fe-Nb alloy system is developed using the simulation approach to predict the phase relationship during solidification. The simulation results can be used as inputs for freckling criteria.;The sensitivity of freckling criteria relies on the accurate acquisition of the attributes that are critical to the key control parameters. These attributes include interdendritic liquid composition, temperature and fraction of solid. Two techniques are developed in the present study to obtain their correlation. From here, liquid composition and fraction solid at any temperature in the mushy zone can be obtained.;Composition effects on the key control parameters are investigated in detail. Liquid density during solidification depends on the composition and the temperature of the liquid. A significant temperature effect on density change is found. Regression analysis shows that Nb has more pronounced effect than Ti in reducing the fraction solid. The dendrite arm spacing and the solidification front angle are largely dependent on the processing conditions.;Freckling criterion in remelting of superalloys is developed at the end of this study. A composition related is proposed to account for the alloying effect on freckling tendency. The model correctly predicts the freckling pattern in Ti- and Nb-containing model alloys. It also correctly predicts the freckling tendency of industrial alloys.

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